Many products, particularly food products are sensitive to the presence of oxygen and/or the loss or absorption of water. These products are susceptible to deterioration when packaged due to oxygen and/or moisture absorption or loss through the wall of the package. Attempts to solve the problem have led to the widespread use of oxygen barriers and/or moisture barriers in packaging materials. Typical moisture barriers include polyethylene and polypropylene. Suitable oxygen barriers include EVOH PVOH, Nylon and blends thereof. Vinylidene chloride--vinyl chloride copolymers and vinylidene chloride--methyl acrylate copolymers are suitable as both moisture and oxygen barrier.
A problem with conventional barrier materials is that due to their high cost or their unstable structural characteristics or other weaknesses, it is difficult to fabricate commercial packaging solely out of barrier materials. For instance, EVOH, while having superior oxygen barrier properties, suffers from moisture problems because of the many hydroxyl groups in the polymer. Other barrier materials are so expensive that to manufacture structures solely from those barriers would be cost prohibitive. Accordingly, it has become a common practice to use multilayer structures whereby the amount of expensive or sensitive barrier material may be reduced to a thin layer and inexpensive polymer can be used on one or both sides of the barrier layer as structural layers. In addition, the use of multilayer structures permits the barrier layer to be protected from attack by stronger structural layers on one or both sides of the barrier layer.
Although multilayer structures containing a barrier layer may be cheaper and stronger than a single layer of barrier material, such structures are more complicated to manufacture than single-layered ones. In addition, multilayer structures being comprised of layers of a variety of different materials may be opposed in some instances on environmental grounds because they may be more difficult to recycle since it is frequently difficult to separate the layers, except at great expense. In addition, reducing the thickness of the barrier layer in a multilayer structure in many instances reduces the barrier properties of that film. Accordingly there is a need for a single-layer packaging material with barrier properties but without the cost, or structural weaknesses of packaging made solely from a barrier material. There is also a need for additional multilayer structures having improved barrier properties where the amount of barrier material may be reduced to a thinner layer than otherwise used and replaced in part by inexpensive structural layers to provide structures that give the same barrier properties of prior art barriers but at lower cost due to a decrease in the amount of expensive barrier material needed.
In addition to barrier properties it is frequently desirable to use materials which have oxygen absorption capabilities. These oxygen absorption or oxygen scavenging materials are useful in reducing the amount of oxygen that contaminate the product packaged in the container. An example of oxygen scavenging materials and methods of using them is disclosed in U.S. Pat. No. 4,425,410 to Farrell et al, the disclosure of which is hereby incorporated by reference herein. Another useful aspect of oxygen absorbing material is that such materials can reduce residual oxygen which is trapped in the headspace of a container during sealing thereby preventing it from having a deleterious effect on the packaged products.
A material that is commonly used in packaging applications is polyethylene terephthalate resin, hereinafter referred to as PET. While PET has a number of valuable properties in packaging, it does not have as good a gas barrier property as is frequently required or desired in many applications. For example, although PET has good oxygen barrier properties for soft drinks, it has not been found useful in packaging such products as beer because beer rapidly loses its flavor due to oxygen migration into the bottle or for such products such as citrus products, tomato based products and aseptically packed meat. A packaging material with physical properties similar to PET is polyethylene naphthalate (PEN), but the latter has a 3-20 times improvement in barrier property and is considerably more expensive.
In order to enhance polyester's gas barrier properties, polyesters have been used in a multilayer structure in combination with a layer having excellent gas barrier properties such as EVOH. One difficulty that has been encountered in multilayer structures employing polyester such as PET is that there are frequently adhesion problems of the polyester to the barrier layer thus leading to possible delamination over time.
One approach to enhancing the gas barrier property of PET is to use a resin mixture which includes PET and a xylylene group containing polyamide resin. Such resin materials are disclosed e.g. in U.S. Pat. No. 4,501,781 to Kushida et al. One of the considerations encountered with such blends according to Kushida is that there is a limit to the amount of xylylene group-containing polyamide resin that may be present in the PET blend. Kushida indicates that amounts of xylylene group-containing polyamide resin greater than 30% by weight with PET causes the container to become a laminated foil structure thereby possibly causing exfoliation between the foil layers of the container.
According to Kushida, the permeation of oxygen gas in a container made with PET and a xylylene group-containing polyamide decreases when compared with the permeation of oxygen gas through a container made solely of PET. Kushida reports that a bottle shaped container made with PET-xylylene group-containing polyamide had 0.0001 cc of oxygen permeation per day compared to 0.0180 cc of oxygen permeation per day for a container molded with PET.
While this patent reports improved barrier properties using PET nylon MXD6 blends for oxygen there is a further need for oxygen barriers of greater resistance to permeation. In addition, there is a need for compositions which exhibit oxygen absorbing properties in addition to acting as a good oxygen barrier material. Thus it is an object of the present invention to provide an improved monolayer barrier structure that satisfies both barrier and absorption functions.
It is also an object of the present invention to provide a monolayer barrier structure of the present invention that has superior barrier properties than known barrier materials.
It is a further object of the present invention to provide a multilayer structure having a PET layer with improved oxygen scavenging ability.